1. Field of the Invention
The present invention relates to a voltage-controlled oscillator which may be used in mobile telephones and other apparatus.
2. Prior Art
Conventionally, in a voltage-controlled oscillator of this kind (hereinafter called VCO), a dielectric resonator and so forth using ceramic has been employed as an impedance element in a resonant circuit. Recently, however, together with the requirement of miniaturization of a mobile telephone set and so forth, a mobile telephone set which uses a surface acoustic wave resonator (hereinafter, called a SAW-R) as an impedance element in its resonant circuit is being developed.
FIG. 6 is a circuit diagram showing an example of the constitution of a VCO using a SAW-R. In FIG. 6, reference numeral 1 is a control voltage input terminal, 2 is a choke coil, 3 is a variable capacitance diode, 4 is a capacitor, 5 is a SAW-R, 6 is an expansion coil, 7 is a DC blocking capacitor, 8 is a transistor, 9 and 10 are feedback capacitors, 11 is an earthing capacitor, 12 is an output capacitor, and 13 is an output terminal. With such a constitution, when a voltage applied to the control voltage input terminal 1 is varied, the capacitance of the variable capacitance diode 3 varies, and a resonant frequency of a resonant circuit composed of the expansion coil 6, the SAW-R 5, the capacitor 4, and the variable capacitance diode 3 varies in response to the variation in the capacitance of the diode 3, by which an oscillation frequency of the VCO varies.
FIG. 7 shows an equivalent circuit of a SAW-R, and FIG. 8 shows a reactance characteristic (solid line Al) of the SAW-R. In FIG. 8, the abscissa shows a frequency, and the ordinate shows a reactance. FIG. 8 shows that the characteristic in the upper portion of the abscissa is inductive and that the lower portion of the abscissa is capacitive. The frequency fs is the frequency at which the SAW-R comes into a series resonant state, and the frequency fp is the frequency at which the SAW-R comes into a parallel resonant state. In the case where the series circuit of the variable capacitance diode 3 and a SAW-R 5 resonates, the width of change of capacitance of the variable capacitance diode 3 is represented by .DELTA.C as shown in FIG. 8, the width of change of inductance of the SAW-R 5 is .DELTA.L as shown in FIG. 8, and the width of change of an oscillation frequency of the VCO is .DELTA.F as shown in FIG. 8.
In a mobile telephone and so forth, an oscillation frequency of a VCO ranges over 800 to 900 MHz, a frequency deviation 25 to 33 MHz is required as a width of change of a frequency, and a value not less than a fixed value is required as a C/N (carrier signal level/noise level) characteristic.
In order to make the width of change .DELTA.f, the expansion coil 6 shown in FIG. 6 (or a microstrip line) has been connected in series with the SAW-R 5. When the expansion coil 6 is inserted, a reactance characteristic of the SAW-R 5 plus the expansion coil 6 is like a broken line A2 as shown in FIG. 8, the series resonant frequency fs lowers, and the width of change .DELTA.f of an oscillation frequency of the VCO enlarges to .DELTA.f.sub.1. However, there has been a problem that when a coil having a large inductance is used as the expansion coil 6, Q of the resonant circuit lowers and consequently, the ratio C/N lowers.